The present invention is directed to method and apparatus for dielectrophoresis that eliminates the use of conventional embedded electrodes and substitutes in their place insulating flow structures that create spatially inhomogeneous electric fields that effect dielectric transport. Further, the use of these flow structures provides for dielectrophoretic separations by means of steady applied electric fields, applied alternating electric fields, or combinations thereof.
Electrokinesis and dielectrophoresis are two technologically important particle and fluid transport mechanisms in microscale flow channels. In the former, particle or fluid transport is produced by an applied electric field acting on a fluid or particle immersed in a fluid having a net mobile charge and is widely used as a mechanism for manipulating particles and conveying fluids in Microsystems.
Dielectrophoresis is particle motion produced by an electric field gradient on the induced dipole moment of a particle and the surrounding fluid. Rather than being linear in the applied field as is the case with electrokinesis, the dielectrophoretic potential field experienced by a particle is second order in the local electric field and is proportional to the difference between the particle and fluid polarizabilities. As disclosed, for example, in U.S. Pat. No. 3,162,592, “Materials Separation Using Non-uniform Electric Fields”, issued to Pohl; U.S. Pat. No. 5,814,200, “Apparatus for Separation by Dielectrophoresis”, issued to Pethig et al.; U.S. Pat. No. 4,326,934, “Continuous Dielectric Cell Classification Method”, issued to Pohl; U.S. Pat. No. 6,071,394, “Channel-less Separation of Bioparticles on a Bioelectronic Chip by Dielectrophoresis”, issued to Cheng et al.; 5,888,370, “Method and Apparatus for Fractionation Using Generalized Dielectrophoretic and Field Flow Fractionation”, issued to Becker et al., dielectrophoresis finds extensive application in manipulating, fusing, sorting, and lysing biological materials. However, prior art dielectrophoretic applications have the disadvantage that they require not only the use of networks of embedded electrodes that can be difficult and costly to fabricate to accomplish the desired result but also application of an alternating electric field having zero mean value. Because prior art dielectrophoretic separations apparatus depends upon the use of an applied electric field to effect separation, fluid flow through such an apparatus must be pressure-driven. Electrokinetic or electric field-driven flow cannot be used because of interferences with the electric field produced by the embedded electrodes and its attendant effect upon the separations process. Pressure-driven flow produces more hydrodynamic dispersion of an analyte than electrokinetically driven flow. Moreover, the prior art employs electrodes that produce field gradients in three dimensions, e.g., electrodes deposited on the top, bottom, or both surfaces of a channel. The dielectrophoretic effect decreases away from these electrodes. This decrease limits the maximum depth of the channels over which dielectrophoresis is effective. Channels cannot be made arbitrarily deep to support a desired volumetric flow rate or sample throughput. The depth dependence of the dielectrophoretic effect is also a source of analyte dispersion in a separation.